Referring to FIGS. 1 and 2, a conventional multiuser wireless (wireless) system 100 is shown as well as user equipment 200. Wireless system 100 includes first transmission cell area (cell area) 10a, and second cell area 10b, though, as one of ordinary skill in the art can appreciate, there are normally many more cells in a conventional wireless system 100. In the wireless system 100, first and second user equipment 4a and 4b include transmit antennas 6a and 6b respectively, as well as receive antennas 8a and 8b, respectively. First cell tower 2a transmits first desired transmitted signal 202a to first user (mobile station (MS)) 4a, and second desired transmitted signal 202b to MS 4b. In addition, first cell tower (base station (BS)) 2a receives first desired received signal 204a from first MS 4a, and second desired received signal 204b from second MS 4b. Second BS 2b, in second cell 10b, transmits third desired transmitted signal 202c to third user 4c, and receives third desired received signal 204c from third MS 4c. The nomenclature “desired” refers to those signals that are meant to be received by the particular user equipment, and do not include, as shown in FIG. 1, un-desired signals that can include, among other types, co-channel interference (CCI) signals 14 and adjacent channel interference (ACI) signals 12. From here-on in, the signals transmitted by, and received at user equipment 4 shall be simply referred to as “transmitted signal 202” and “received signal 204.”
As is well known, many different signals impinge upon receive antennas 8a,b, including both desired signals (received signals 204a,b) and signals that are not desired, the above referenced interference signals. All of the received signals combine to form a composite signal 206 (shown in FIG. 2). Composite signal 206 is received by receive antenna 8 (although shown as two separate antennas in FIG. 1, for convenience, those of ordinary skill in the art can appreciate that transmit and receive functions can be performed by a single antenna, as shown in FIG. 2) undergoes down-conversion in RF front end 208, and various stages of decimation and low-pass filtering in the RF before it is available at baseband block 214 (i.e., RF front end 208 down converts the RF signals to an intermediate frequency (IF), which is then received at IF block 210, and the intermediate down-converted received signal is sent to back end block 212, which outputs a baseband signal (i.e., voice, data, video). Due to the low-pass filtering of composite signal 206, the interference signals 12, 14 at antenna(s) 6, 8 are filtered out before reaching the baseband frequency range. The elimination of the interference signals, however, presents a problem for many applications such as automatic gain control (AGC), and determination of the signal-to-interference and noise (SINR) computations, among others, where an accurate computation and/or estimation of the carrier and interference power at the receive antenna 8 is necessary.
In multi-user scenarios, the interference environment can be broadly classified into three categories: sensitivity, co-channel interference (CCI), adjacent channel interference (ACI). In the sensitivity scenario, the input signal is mainly influenced by additive white Gaussian noise (WGN) and the contribution from interference is absent. In the CCI scenario, the interference is from a cell using the same frequency as the input/desired signal. Referring back to FIG. 1, CCI signal 14 could be generated from third MS 4c that is operating on channel 1, which is the same channel that first MS 4a is operating on. The third category of interference is ACI, in which case the interference is due to leakage of power from an adjacent user equipment whose operating channel is close in frequency to the first user equipment. That is, referring to FIG. 1, ACI signal 12 results from second MS 4b operating on channel 2 that is relative close in frequency to channel 1 used by first MS 4a, and wherein, typically, second MS 4b is probably operating in close physical proximity to first MS 4a. 
As discussed above, determination of the power of interference signals is useful for AGC and SINR determination. A brief discussion of the determination of both is relevant for the purposes of this disclosure.
Automatic Gain Control
The total power received in the antenna terminal is the sum of desired carrier power and all the interferer signals power.
                                          Total            ⁢                                                  ⁢            Power            ⁢                                                  ⁢            at            ⁢                                                                      ⁢                                                                    ⁢            Antenna            ⁢                                                  ⁢            Input                    =                                    Carrier              ⁢                                                          ⁢              Power              ⁢                                                          ⁢              C                        +                                          ∑                k                                                                              ⁢                                                          ⁢                                                k                  th                                ⁢                                                                  ⁢                Interferer                ⁢                                                                  ⁢                Power                ⁢                                                                  ⁢                                  (                                      I                    k                                    )                                                      +            wgn                          ,                            (                  Eq          .                                          ⁢          1                )            where wgn is white Gaussian noise.
The received total power at antenna input is input to lower noise amplifier (LNA), as shown in FIG. 2. The gain of LNA 216 is set according to the average received signal strength indicator (RSSI), as computed by the AGC algorithm. The gain determined by the AGC algorithm needs to be set appropriately to avoid as much saturation or clipping of the input signal as possible, while simultaneously ensuring that the received signal is amplified to an optimum level to improve the sensitivity level. Therefore appropriate and fast gain control is very important for receiver signal processing.
SINR Computation
The radio link quality is directly influenced by the amount of interference signal present in the received signal. The greater the amount of interference power present in the received signal, the poorer is the receiver bit error rate (BER) performance. Therefore, there is a need to estimate the proper ratio of carrier-to-interferer (C/I) power.
The SINR is defined as:
                    SINR        =                                            Carrier              ⁢                                                          ⁢              Power                                                      Interference                ⁢                                                                  ⁢                Power                            +                              Noise                ⁢                                                                  ⁢                Power                                              .                                    (                  Eq          .                                          ⁢          2                )            
In a receiver, the SINR is used for various applications, such as equalizers, radio link adaptation, AMR codec rate adaptation, and burst validity checks. Equalizers with different computational complexity can be used based on the estimated SINR. For example, if it can be determined that the SINR is expected to be good (i.e., relatively high), then a low complexity equalizer (e.g., an equalizer that uses only a single antenna) can be used to obtain the desired performance. If, however, it can be determined that the SINR is expected to be poor (i.e., relatively low), a more computationally intensive equalizer (e.g., a multiple antenna equalizer) can be used to obtain better performance.
In radio link adaption, the mobile station (MS) forwards the estimated SINR to the base station (BS), wherein the BS then adjusts the modulation and coding schemes to adjust the data rate accordingly. If the SINR is predicted to be high, a higher data rate can be expected, and if the SINR is expected to be lower, then the data rate is adjusted downward, accordingly.
In global system for mobile communications (GSM) systems, the adaptive multi-user rate (AMR) audio codec rate is adjusted according to the estimated SINR. As known by those of ordinary skill in the art, AMR audio codec is an audio data compression scheme optimized for speech coding. AMR uses link adaptation to select from one of eight different bit rates based on link conditions. AMR utilizes different speech coding techniques depending on circumstances and/or system design. When AMR is implemented, an optimized link adaptation is used to select the best codec mode to meet the local radio channel and capacity requirements. For example, if the radio channel between the BS and the MS is experiencing higher than normal noise (i.e., decreased SINR), source coding is reduced and channel coding is increased. This has the effect of improving the quality and robustness of the network connection, but some voice clarity is sacrificed. Conversely, if radio channel conditions improve, i.e., a high SINR, then source coding can be increased and channel coding decreased.
In order to achieve higher data transmission rates (DTR), the condition of the radio channel between the BS and the MS should be at least at some nominal, or threshold level to ensure that the higher DTR can be accomplished. Therefore, the SINR value is used to indicate the validity of the received burst by comparing the SINR with a threshold value. If the SINR is equal or greater than the threshold, then the burst transmission can be presumed to be validly received.
For at least the reasons discussed above, it is important to estimate the SINR accurately in different channel conditions and environments taking all the factors into consideration for proper receiver operation. Unfortunately, currently employed systems and methods fail to adequately achieve accurate SINR estimations.
As discussed above, the proper estimation of interference power is important to estimate the total power and to estimate the SINR. There are various baseband algorithms/methods available and employed in the receiver to estimate the interference power. They suffer from problems that are described below, in conjunction with AGC and SINR estimation.
As briefly discussed above, the determination of the AGC setting involved a determination of the SINR. In current use, many receivers estimate the total received power using the received signal strength indicator (RSSI) that is computed as:
                              RSSI          =                                                    ∑                m                                                                              ⁢                                                          ⁢                                                I                  ⁡                                      (                    m                    )                                                  2                                      +                                          Q                ⁡                                  (                  m                  )                                            2                                      ,                            (                  Eq          .                                          ⁢          3                )            wherein m is the number of complex (I,Q) samples in the received frame/burst. For example, in GSM, the number of complex samples is 156. Based on the RSSI value, an appropriate gain is selected for the received signal attenuation/amplification. But, when the composite signal passes through the RF front-end circuit, some of the signal is filtered out (low pass filtering) and attenuated by the RF circuit. This is not directly visible in the baseband where RSSI is computed. If there are strong ACI signals present in the input signal, then a major part of that would be filtered out by the RF low pass filters due to which the estimated in-band RSSI in the baseband would be much lower than the true RSSI at the antenna. This can lead to incorrect AGC gain being applied leading to undesirable effects like saturation or analog-to-digital converter (ADC) clipping that can severely degrade the receiver performance.
When performing SINR estimation, interference is normally estimated based on either pilot/training symbols or data symbols. Since the interference power reaching the baseband is already filtered out or attenuated by the RF block, the estimated interference power is often inaccurate leading to incorrect SINR estimation.
In many GSM receivers the SINR is also estimated using the soft-bit quality (SFQ). A significant issue in this approach is that in the high and low SINR region, the SFQ based SINR saturates. That is, the estimated SINR does not change even though the actual SINR at the antenna changes. No differentiation of SINR value is noticeable in these regions (high and low) and hence SINR cannot be calculated accurately for the high and low SINR regions.
Accordingly, it would be desirable to provide methods, modes and systems for accurately and effectively determining carrier and interference powers at the antenna before signal processing can alter the fundamental nature of the received signal irretrievably.